FPU serialization uses
preempt_disable(); local_bh_disable();
which is silly because local_bh_disable() implicitly disables
preemption. So this can be reduced to local_bh_disable().
But this does not work on RT kernels because local_bh_disable() only
serializes bottom half related processing via a local lock, but does not
disable preemption.
On RT kernels preempt_disable() is the right choice because on RT bottom
half processing is always in thread context, so preempt_disable()
implicitly protects against bottom half processing there.
Thanks,
tglx
